This invention relates to a speed fastener for securing apertured workpiece members together.
Speed Fastening® is a well known method of securing workpiece members together, whereby a fastener, such as a rivet, having a hollow core is placed in aligned apertures in the workpiece members, and a headed mandrel is pulled through the core of the fastener to cause radial expansion of the fastener shank, and additionally a small degree of foreshortening of the fastener. The radial expansion of the fastener shank ideally achieves hole filling of the holes in the workpiece members. The axial foreshortening effect can help pull the rearmost sheet up towards the fastener head to close gaps between the workpiece members.
Prior art fasteners, such as those available under the trademarks Briv® (as described in UK patent number GB1323873) and Chobert®, have shank portions which in the centre region have a uniform wall thickness. In both cases, the tail end of the fastener shank has an increased wall thickness; in the case of the Briv fastener this is an enlarged outer diameter and in the case of the Chobert fastener, a reduced bore diameter of tapered form.
These prior art fasteners are installed by pulling a mandrel of a given diameter through the fastener bore, thereby causing a radially enlarged head of the mandrel to expand the central region of the shank into the apertures in the workpiece members. Accordingly, the degree of radial expansion is governed by the diameter of the mandrel head for a nominal fastener diameter.
Prior art fasteners have a low degree of tolerance in workpiece aperture diameter, i.e. only a limited range of workpiece aperture diameters can be accommodated with a nominal fastener size and a single head diameter mandrel. Accordingly, there is a narrow range between the two acceptable extremes of workpiece aperture diameter when using a nominal fastener and a specific head diameter mandrel.
In an optimised installation of a prior art fastener, i.e. wherein the workpiece aperture diameters are within the acceptable range for a given fastener diameter and mandrel head diameter, the expansion of the fastener shank by the mandrel head causes mechanical engagement between the fastener shank and the walls of the workpiece apertures, without ‘overpacking’ (as explained below). The constraint provided by the wall of the aperture of the rear workpiece member (i.e. the workpiece member which is furthest away from the fastener head) generates radial forces and hence frictional resistance to axial movement of the rear workpiece member relative to the fastener shank.
However, if a workpiece aperture diameter is smaller than the acceptable range for a prior art fastener, an unduly high placing force will be required to install the fastener because it will be overpacked in the aperture. This can cause wear of the mandrel or cause other undesirable effects such as debris generation from the fastener bore, or fastener head malformation, such as by extrusion of fastener bore material up into the head region.
It is common for the aperture diameter of the top workpiece member (i.e. the workpiece member which is closest to the fastener head) to be configured such that it is slightly larger than the aperture in the rear workpiece member, in order to accommodate aperture pitch errors. If the aperture diameter of the top workpiece member is above the acceptable range for a prior art fastener, the expansion of the shank of the fastener does not result in any mechanical engagement between the fastener shank and the wall of the oversized workpiece aperture, resulting in low resistance of the joint to movement under shear loads.
To compensate for oversize workpiece apertures, an oversized mandrel, i.e. a mandrel having an oversized head, can be used to install a fastener. However, there is a risk that an oversized mandrel can incorrectly be used for a undersized workpiece aperture, thereby leading to the problems caused by a high placing force as discussed above.
The problem of a workpiece aperture diameter being above the acceptable range for a specific fastener diameter and mandrel head diameter is illustrated in FIG. 1, which illustrates a prior art fastener 2 installed into a workpiece 30 at the lowest end of a grip range. The workpiece comprises a top workpiece member 32 having an oversized aperture 36, and a rear workpiece member 34 having an acceptable aperture 38. Due to the top workpiece member 32 having an oversized aperture 36, the installation has not resulted in any mechanical engagement between the fastener shank 4 and the wall of the oversized aperture 36.
A correctly installed prior art fastener exhibits a enlarged tail end formation due to the greater wall thickness in that region. In a maximum-grip application, this enlarged tail end formation is adjacent to the outer surface of the rear workpiece member and so serves to provide support and resistance to movement of the joint if subject to tensile loads. However in a mid-grip or a minimum-grip condition (as shown in FIG. 1) the enlarged tail formation 44 is remote from the rear workpiece member 34 and it is therefore unable to help resist initial separation of the workpiece members 32, 34 under tensile loads; the resistance must result almost entirely from the frictional forces resulting from the radial pressure exerted by the expanded central region of the fastener shank 4 having a uniform wall thickness. Consequently the resistance to separation is lower in mid or minimum-grip conditions.
Other speed fasteners which are currently available, such as those available under the trade marks Avtronic® and Rivscrew®, feature a non-uniform wall thickness in the centre of the fastener shank. These fasteners comprise voids on the exterior of the fastener shank in the form of annular grooves and a helical screw thread form respectively. These fasteners are intended to expand and embed into a workpiece wherein the workpiece members are comprised of a material which is relatively soft in comparison to the material of the fastener. The annular or helical voids of these fasteners lead to variable expansion into the apertures of the workpiece members, with high or low degrees of mechanical engagement between the fastener shank and the wall of the workpiece apertures, and accordingly a consistent and sufficient engagement between the fastener shank and the wall of the workpiece apertures cannot be guaranteed. Furthermore, these prior art fasteners result in an expanded tail form of inconsistent size or distribution around the hole periphery.
The present invention is aimed at providing a fastener which will at least mitigate the above problems, and ensure complete and consistent mechanical engagement between the fastener shank and the wall of both the workpiece apertures, i.e. achieve complete mechanical engagement for a fastening which is more robust than is achievable with prior art fasteners. The present invention is also aimed at providing a fastener which allows the specification of a single fastener length and one mandrel size to install into a wider grip range and which can tolerate a far greater variation in workpiece aperture diameter than prior art fasteners, whilst avoiding the problems of high placing force, mandrel wear, debris generation and fastener head malformation with a single mandrel head size or diameter.
Furthermore the present invention is aimed at providing a fastener which is suitable for use in securing workpiece members which may be of harder material than the fastener, and wherein the grip range of the fastener is dependent only upon the length of the fastener i.e. a longer fastener has a greater grip range, allowing a fastener having a nominal length to be used in a wider range of workpiece thicknesses.
Accordingly the present invention comprises, in a first aspect, a fastener for securing a workpiece comprising a plurality of workpiece members, said fastener comprising a shank with a head end and a tail end remote from the head end, and a radially enlarged head at the head end of the shank, and a bore extending throughout the fastener, wherein an internal wall of the shank comprises a voided section comprising a plurality of voids separated by a plurality of splines each of which culminates in a crest, the voids and splines being elongated in an axial direction with respect to a longitudinal axis of the fastener bore; wherein the splines are of an approximate trapezoidal cross-section.
The present invention comprises, in further aspects, a method of installation of a fastener comprising the steps of placing a mandrel having a radially enlarged head through the fastener bore such that the radially enlarged head of the mandrel is adjacent the tail end of the fastener shank, inserting the fastener and mandrel into the apertures in the first and second workpiece members such that the fastener head contacts the first workpiece member, and supporting the fastener at the head end whilst drawing the mandrel entirely through the fastener bore, thereby expanding the fastener shank into the apertures of the workpiece members, and causing the crests of the splines to be deformed, and causing the tail end of the fastener shank to radially enlarge adjacent to the second workpiece member. The present invention also comprises, in further respects, an apparatus for securing a workpiece comprising a plurality of workpiece members, said apparatus comprising a fastener and a mandrel; said fastener comprising a shank with a head end and a tail end remote from the head end, and a radially enlarged head at the head end of the shank, and a bore extending throughout the fastener; wherein an internal wall of the shank comprises a voided section comprising a plurality of voids separated by a plurality of splines each of which culminates in a crest, the voids and splines being elongated in an axial direction with respect to a longitudinal axis of the fastener bore; wherein the splines are of an approximate trapezoidal cross-section.
The installation of the fastener may be undertaken by a tool comprising a conical concave end face which bears on an upper surface of the fastener head. This causes the fastener head to be pushed towards the top workpiece member and the head to flatten slightly against the top workpiece member, and thereby ensuring clamping of the fastening. The flattening of the head periphery against the top workpiece member acts such that the outer diameter of the underhead recess decreases, and the bearing area of the fastener head against the workpiece is increased. Furthermore the bearing area of the tool end face on the upper surface of the fastener head is greater than with prior art speed fasteners such that undesirable indentations or damage to the surface of the fastener head are minimised during installation of the fastener.
The present invention comprises, in further aspects, a method of installation of a fastener comprising the steps of inserting a stem comprising a radially enlarged head and a breakneck point into the fastener through the fastener bore such that the radially enlarged head of the stem is adjacent the tail end of the fastener shank; inserting the fastener and the stem into the apertures in the first and second workpiece members such that the fastener head contacts the first workpiece member; supporting the fastener at the head end whilst pulling the stem with respect to the fastener head, thereby causing the stem head to enter the tail end of the fastener shank thereby radially expanding the fastener shank into the apertures of the workpiece members, and causing the crests of the splines to be deformed against the stem head, and causing the tail end of the fastener shank to radially enlarge adjacent to the second workpiece member; wherein the stem is pulled with respect to the fastener head through the fastener bore until the stem fractures at the breakneck point. The present invention comprises, in yet further aspects, a method of installation of a fastener comprising the steps of inserting the fastener into the apertures in the first and second workpiece members such that the fastener head contacts the first workpiece member; inserting a breakstem comprising a plugging portion with a hollow core, an elongate shank, and a breakneck point between the plugging portion and the elongate shank, into the bore of the fastener, such that the plugging portion of the breakstem contacts the tail end of the fastener shank; pulling the elongate shank of the breakstem pulled relative to the fastener, thereby causing the breakstem plugging portion to enter the fastener shank, causing the fastener shank to expand into the workpiece apertures, and causing the crests of the splines to be deformed against the plugging portion, and simultaneously causing the plugging portion to collapse inwardly; wherein the elongate shank is pulled relative to the fastener until the breakstem fails at the breakneck point.
A fastener installed by the disclosed methods may further comprise a parallel portion between the breakneck point and the plugging portion, which, during installation, forms a locking skirt thereby providing a mechanical lock of the installed stem in the fastener shank.
The present invention comprises, in a further aspect, a method of installation of a fastener comprising the steps of inserting the fastener into the apertures in the first and second workpiece members such that the fastener head contacts the first workpiece member; and driving a solid pin, having a maximum diameter larger than that of the fastener bore, into the bore of the fastener via the head thereby to radially enlarge the fastener shank into mechanical engagement with the workpiece apertures, and causing the crests of the splines to be deformed against the solid pin. This method may further comprise a subsequent step of bearing on the fastener head by a support sleeve, and removing the pin from the fastener bore.
Pulling the mandrel head, or pushing a solid pin, through the bore causes the fastener shank to increase in diameter to engage mechanically with the wall of the workpiece apertures.
The axial voids of the present invention allow a variable degree of radial crushing of the fastener bore splines by the mandrel head as it is pulled through different sizes of workpiece member aperture, such as an oversized aperture in the top workpiece member, thereby providing mechanical resistance to shear movement, without leading to ‘overpacking’, thereby avoiding potentially excessive placing loads, mandrel wear, debris generation and fastener head malformation. The outer surface of the shank in contact with the wall of the workpiece apertures is far greater than that obtained by fasteners having helical threads or annular grooves on the fastener shank, and so stresses are lower when the joint is loaded subsequently.
Because the voids are positioned axially with respect to the fastener shank, they have a cross-sectional area which is constant along the length of the fastener shank. This ensures that the degree of radial expansion and the mandrel load are consistent, independent of grip thickness. The present invention can therefore be used in a greater grip range than that offered by equivalent-sized prior art fasteners. Furthermore the grip range which can be accommodated by a specific fastener is dependent only upon the length of the fastener, i.e. a longer fastener can accommodate a greater grip range.
Furthermore, the present invention provides that a single mandrel (i.e. a mandrel having a set head size) can be used to install a fastener into a workpieces having a variety of aperture sizes.
The present invention also provides greater expansion of the tail end of the fastener shank adjacent to the rear workpiece member, throughout the grip range, thereby providing increased resistance to separation of the workpiece members under tensile loads.
The present invention is also suitable for use in securing workpiece members which may be of harder material than the fastener.
The voids are preferably equidistant from one another.
The fastener head may include a counterbore in its top surface, the counterbore having an average diameter which is greater than the minor diameter of the bore of the fastener. The counterbore could be parallel to the longitudinal axis of the fastener bore, or could comprise a flat or a curved annular tapered wall, and a tapered transition portion could be provided between the counterbore and the fastener bore, wherein the spline minor diameter increases towards the counterbore. The counterbore and the tapered transition portion both accommodate axial spline material which is drawn axially through and into the fastener head by the passage of the enlarged mandrel head through the fastener bore. Furthermore, the counterbore controls the broach load on installation of the fastener, and avoids excess spline material pulling out from the top of the installed fastener head.
The tail end section of the fastener may include a tapered point. An advantage of the tapered point is that it allows fasteners to be nested together “head-to-tail” in a stack within a paper pod, etc, when a counterbore is provided in the fastener head. This ensures coaxial alignment of adjacent fastener bores which in turn makes it easier to load the fastener stack as one onto a mandrel shank. This stack may be packaged in a paper “pod” for ease of handling, the pod being easily stripped off the stack by hand once it has been loaded onto the mandrel. Furthermore the tapered point also assists in locating the fastener in the workpiece aperture, and reduces the height of the stack of fasteners prior to installation, thus permitting more fasteners to fit a placing tool of a given length, and provides increased rigidity to the stack on the relatively flexible mandrel.
The end section of the fastener may also include a solid tail ring, i.e. wherein the voided section does not extend into the tail ring. The tail ring, despite its remote position from the rear workpiece member, nonetheless contributes to the ultimate tensile strength of the fastener via its reinforced wall section.
In addition to the tapered point, the end section of the fastener may further comprise a “dog point”, i.e. a straight annular wall which is parallel to a longitudinal axis of the fastener, wherein the dog point is located further away from the splined section than the tapered point. In an embodiment of the present invention wherein the counterbore is parallel to the longitudinal axis of the fastener bore, and the diameter of the dog point is similar to that of the counterbore, fasteners can be nested together “nose to tail” in the delivery format within the paper pod, etc, i.e. the dog point and counterbore facilitate the storing of multiple fasteners “head-to-tail” in a stack prior to installation, whereby the increased mechanical engagement of the dog point into the adjacent fastener head counterbore further enhances the coaxial alignment of adjacent fastener bores and greatly increases resistance to bending of the stack.
The axial voids of the present invention allow greater expansion of the fastener shank beyond the rear workpiece member, and also allow total aperture filling by the fastener shank in different sizes of workpiece aperture, (i.e. wherein the after installation of the fastener the aperture is completely filled with fastener shank material), without leading to “overpacking”, thereby avoiding excessive broach loads and excessive stresses in the workpiece apertures.
Furthermore the present invention generates an enlarged tail end formation adjacent to the rear workpiece member throughout a grip range, and the grip range throughout which the fastener can be used is greater than that offered by equivalent-sized prior art fasteners.
In the present invention, the fastener shank has a central region featuring an increased wall thickness which incorporates a plurality of axial “voids”. The axial voids extend radially outwards from the minor diameter of the shank bore.
Inside the head the fastener bore is of greater diameter than the shank bore minor diameter, and there is a tapered transition region between the two.
The radial expansion of the fastener shank beneath the fastener head is sufficient to create hole filling in joints where the aperture in the top workpiece member is larger than the aperture in the rear workpiece member and so there is mechanical resistance to shear movement.
The voids of the present invention are more substantial in size than those provided in prior art fasteners such as Avtronic and Rivscrew and are orientated axially, this providing a cross-sectional area that is constant along the length of the fastener shank where it expands into the rearmost workpiece member. This ensures that the degree of radial expansion and the mandrel load is consistent, independent of grip thickness.
The present invention provides a greater grip range and a greater tolerance to variation in aperture diameters than prior art fasteners. Furthermore, the fastener ensures hole-filling into oversized apertures in the top workpiece member, resulting in significant benefit in increasing joint stiffness in shear.
The enlarged tail formation adjacent the rearmost workpiece member is available throughout the grip range and offers greater mechanical resistance to joint separation under tensile loads.